Change categories validation script to use Ruby instead of Python

What does this MR do and why?

WHAT

• This MR changes the scripts/lint/validate_release_categories.py to scripts/lint/validate_release_categories.rb.
• Related to issue mentioned in References section below.

WHY

• Ruby has more built in functionality to parse YAML which is needed to check frontmatter case (newly introduced in leaf bundles).
• GitLab is primarily written in Ruby so let's keep our scripts in Ruby as well.

References

Related issue: gitlab-org/technical-writing/docs-gitlab-com#672 (comment 3425101833)

How to set up and validate locally

1. Locally change a file under doc/releases to have an error for example. Remove the category tag comment in file doc/releases/19/gitlab-19-1-released/clearer-security-labels.md and add it to frontmatter like the following

---
title: Clearer, security industry-standard labels in vulnerability details
stage: application_security_testing
weight:  
level: secondary
tier: ultimate
offering: [ gitlab_com, self_managed, gitlab_dedicated ] 
documentation_link: "../../../user/application_security/vulnerabilities/"
work_item: "https://gitlab.com/groups/gitlab-org/-/work_items/21978"
categories: [ Vulnerability Management ]
---

In GitLab 19.1, the vulnerability results details page uses consistent, descriptive, and security industry-standard terminology for scan results:

- **Scanner** is now **Detected by**
- **EPSS** is now **Exploit Probability (EPSS)**
- **Has Known Exploit (KEV)** is now **Known Exploited (CISA KEV)**
- **Reachable** is now **Reachability**
- **Image** is now **Container Image** (Container Scanning)
- **Location** is now **Affected Location**
- **URL** is now **Affected Endpoint** (DAST, API fuzzing)
- **Method** is now **HTTP Method** (DAST, API fuzzing)
- **Solution** is now **Remediation Guidance**
- **Links** is now **References**

2. For error scenario put a category that is not real like "BAD CATEGORY":

---
title: Clearer, security industry-standard labels in vulnerability details
stage: application_security_testing
weight:  
level: secondary
tier: ultimate
offering: [ gitlab_com, self_managed, gitlab_dedicated ] 
documentation_link: "../../../user/application_security/vulnerabilities/"
work_item: "https://gitlab.com/groups/gitlab-org/-/work_items/21978"
categories: [ BAD CATEGORY ]
---

In GitLab 19.1, the vulnerability results details page uses consistent, descriptive, and security industry-standard terminology for scan results:

- **Scanner** is now **Detected by**
- **EPSS** is now **Exploit Probability (EPSS)**
- **Has Known Exploit (KEV)** is now **Known Exploited (CISA KEV)**
- **Reachable** is now **Reachability**
- **Image** is now **Container Image** (Container Scanning)
- **Location** is now **Affected Location**
- **URL** is now **Affected Endpoint** (DAST, API fuzzing)
- **Method** is now **HTTP Method** (DAST, API fuzzing)
- **Solution** is now **Remediation Guidance**
- **Links** is now **References**

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